1. Field of the Technology
This invention concerns metal composite materials, and more particularly a laminated composite of a metal powder and a ceramic, glass or glass-ceramic powder bonding agent.
2. Description of the Prior Art
Magnetic materials can be classified into two groups with either hard or soft magnetic characteristics. Hard magnetic materials retain a large amount of residual magnetism after exposure to a strong magnetic field. In contrast, soft magnetic materials become magnetized by relatively low-strength magnetic fields, and return to a state of low residual magnetism when the applied field is removed. Soft magnetic behavior is essential in any application involving changing electromagnetic induction such as solenoids, relays, motors, generators, magnetic bearings, gyroscopes, transformers, and magnetic shielding. The most desirable characteristics in magnetically soft materials include: high permeability, high saturation induction, low hysteresis-energy loss, and low eddy-current loss in alternating flux applications. Other factors such as cost, availability, strength, corrosion resistance, and ease of processing also influence the final selection of a soft magnetic material. Thus, for many applications, the relatively low cost irons or steels are used. Pure iron has a saturation induction of 2.16 Tesla (21.58 kilogauss); a value sufficient for many applications. However, some applications demand the higher saturation values that can only be achieved in alloys of iron and cobalt. The highest known saturation induction value is 2.46 T (24.6 kG) which occurs in an Fe-Co alloy containing about 35 wt. % cobalt. Because of the high cost of cobalt, such high performance magnetically soft materials tend to be used in only the most demanding applications. For example, the majority of transformers, motors, and generators have cores of high-purity iron, low-carbon iron, silicon steel, or nickel-iron alloys. However, for very high performance transformers, transducers, electrical generators, and magnetic bearings, the material of choice for the core laminations is one of the iron-cobalt alloys. According to a chapter in the ASM Metals Handbook (Douglas W. Dietrich in Tenth Edition, Volume 2, Properties and Selection; Nonferrous Alloys and Special-Purpose Materials, p 774), the iron-cobalt alloys are used in radar pulse transformers, transducers, electromagnet pole tips, and high-output power generators. Experience has shown that transformers containing iron-cobalt alloys are generally used in very specialized instruments including those procured by the military. The power generators that use these materials include those in commercial and military aircraft.
The cores of motors, generators, transformers and magnetic bearings are generally comprised of a large number of thin metal laminations that are separated from one another by a layer of insulating material. In the highest performance magnetic cores, the metal laminations are comprised of one of two iron-cobalt alloys (Fe-49Co-2V or Fe-27Co-0.6Cr). The interlaminar insulation, which may consist of an oxide layer on the metal plus an organic adhesive layer between laminations, is necessary to ensure high electrical efficiency in the magnetic core. Presently, the most demanding applications for those core assemblies are those used in airborne power generators.
Airborne power generation requires compact, high-output equipment and thus a lamination material with the highest saturation induction and lowest hysteresis losses, i.e., an iron-cobalt alloy. The high rotational speeds in these devices, on the order of 12,000 rpm, impose significant mechanical stresses on the rotor material as well as the adhesive that bonds the laminates. In fact, the yield strength of the magnetic rotor material may be the decisive factor in alloy selection for this application, and it is highly desirable that the strength of the adhesive bond be comparable with that of the magnetic material.
There are presently under development two new demanding applications for magnetic materials--compact, very high speed electrical generators; and high-temperature magnetic bearings. The proposed generators spin at speeds on the order of 100,000 rpm, resulting in high stresses on the foil laminates and the adhesives joining them.
The high-temperature magnetic bearings are being considered for future gas turbine engines. Magnetic bearings could increase the reliability and reduce the weight of these engines by eliminating the lubrication system. They could also increase the DN (diameter of the bearing times rpm) limit on engine speed, and allow active vibration cancellation systems to be used--resulting in a more efficient, "more electric" engine. The magnetic bearing is similar to an electric motor. It has a laminated rotor and stator, likely made of an iron-cobalt alloy. Wound around the stator are a series of electrical wire coils that form a series of electric magnets around the circumference. The magnets exert a force on the rotor. A probe senses the position of the rotor, and a feedback controller keeps it in the center of the cavity. For gas turbine applications, it is desirable that the magnetic bearings be capable of operating at temperatures on the order of 650.degree. C.
Unfortunately, organic adhesives lose much of their strength at relatively modest temperatures. For example, according to a chapter by John Williams in the ASM Engineered Materials Handbook, Volume 1, Composites, 1987 ("Adhesives Selection,"p. 684) the maximum use temperatures for organic adhesives range from 82.degree. C. for epoxies to 260.degree. C. for some polyimides.
The strength of magnetic rotor assemblies can be enhanced by the addition of metal pins or stakes that are inserted into holes punched in the laminations. However, there is a penalty in electrical efficiency for the use of such devices since the lamination factor (solidity of the core) is reduced when the magnetic lamination material is replaced by a non-magnetic material.
Thus, there is a need for a method to strongly bond together the magnetic laminations of the cores of high performance electromagnetic equipment without the strength and temperature limitations of organic adhesives. Further it would be desirable to provide a method for avoiding the need for mechanical staking presently used to fabricate such magnetic structures, and to replace such method with a stronger, more efficient design.
Another problem occurs when it is desired to fabricate a large magnetic core assembly with the laminations in a "pancake" geometry in order to attain certain magnetic-field effects. In such a case, the diameter of the core is limited by the maximum width of foil available, which for Hiperco 50A is about 40 cm (16 inches). One alternative is to change the coil design to one in which a long, narrow length of foil is wrapped in a spiral configuration, but that is not always desirable from a magnetic performance standpoint.
A further problem associated with metal foil laminations is that certain alloys are too brittle to roll into foils. For, example, the Fe-6.5Si alloy is known to possess excellent magnetic properties (high permeability and low hysteresis losses) but is too brittle to process into foil. Thus, it would be desirable to provide a method for manufacturing high performance magnets in sizes precluded by the available widths of foil, or which would allow the use of alloys formed of compositions that are too brittle to roll into foil.